Climate change is "hanging like a shadow" over all international development efforts, a top UN official says, as the world body moves to tackle global warming in a landmark summit this week that will see more than 120 heads of state attend.

Helen Clark, head of the United Nations development program, made the comments on Sunday afternoon, while more than 100,000 people marched through New York City to call for tougher action on reducing greenhouse gas emissions. The demonstration coincided with dozens of similar events around the globe.

At the same time, a new report underscored the pessimism about reaching a workable climate deal in 2015, noting that carbon dioxide emissions are expected to hit a record high this year.

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UN Secretary-General Ban Ki-moon, who participated in the New York march, has called on global leaders to use Tuesday's climate summit as a platform to announce bold plans for reducing greenhouse gas emissions, outside of the regular negotiation process. But there's widespread skepticism about what the summit can achieve, in part because the leaders of two of the world's largest greenhouse gas emitters – China and India – are not attending the event. Prime Minister Stephen Harper has also opted out of the one-day summit, though he will attend a separate dinner with Mr. Ban and other leaders to discuss climate issues while he's in New York City.

Nonetheless, there is a great deal of anticipation over President Barack Obama's speech to the climate summit, where he is expected to highlight U.S. efforts to curb domestic emissions and call on other countries to set ambitious goals.

Territorial emissions

The meeting comes amid growing concern about the world's ability to limit the increase in average global temperatures to 2 degrees above preindustrial times. A new report published in the journal Nature Geoscience on Sunday says that may not be possible unless more than half of known fossil-fuel reserves are left in the ground. The study also warns that the 2-degree goal is slipping away, as global emissions continue to rise, driven in large part by economic growth in China.

Asked about the impact of the Chinese and Indian leaders' absence, a spokesperson for Mr. Ban said the Secretary-General understands that "diplomatic issues" prevented them from doing so. "He also understands the commitment of the representatives of both those governments to come to the summit with bold proposals, and we look forward to hearing those speeches," Stéphane Dujarric said.

More than 120 heads of state are expected to attend Tuesday's summit, and Mr. Harper's decision not to join them has drawn criticism from opposition MPs and activists at home, who are already worried that Canada won't meet its targets on reducing greenhouse gases. "Everybody else is going to be there," NDP environment critic Megan Leslie said Sunday on CTV's Question Period. "Our absence has already been noticed."

While Mr. Harper is not attending the climate summit himself, his focus on improving maternal and child health in poor countries – a topic he is expected to raise during his speech on Thursday at the UN General Assembly – suggests he may not be able to avoid the issue.

Mr. Harper is also expected to comment on the threat of Islamic State extremism and Russian aggression in Ukraine in his first speech before the General Assembly since 2010.

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Ms. Clark, the UN development program head, said there is a growing view that global development goals, including reducing the burden of disease and improving maternal and child health, can't be achieved without greater action on climate change. "Climate change is hanging like a shadow over every development discussion. It has to be addressed," she said.

Mr. Harper will attend a separate dinner on climate change with Mr. Ban and the leaders from nearly two dozen other countries, including Brazil, Japan, Britain and the United States, according to a draft attendance list obtained by The Globe and Mail.

A spokesperson for Environment Minister Leona Aglukkaq, who will represent Canada at the climate summit in Mr. Harper's place, told The Globe that Ottawa is making "significant efforts" to curb climate change while maintaining economic growth.

However, figures provided by Environment Canada indicate Canada is not on track to meet its commitments under the Copenhagen agreement, which called for a 17-per-cent cut in emissions from 2005 levels by 2020.

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